As shown in FIGS. 1 and 2, in one widely used approach to measuring the liquid level 10 in a closed tank 12, a differential pressure monitoring apparatus 14 includes a differential pressure sensor 16 attached by an umbilical cord 18 to a sensor housing 20 including a sealed closure 22 adapted for attachment to an opening, such as a bung-hole 24, located on an upper surface of the tank 12. The umbilical cord 18 has sufficient length to allow the differential pressure sensor 16 to lie on, or be suspended very close to the bottom of the tank 12 when the closure 22 of the monitor 14 is operatively sealing the opening 24 in the tank 12.
As shown in FIG. 1, the differential pressure sensor 16, of such prior monitoring apparatuses 14, have typically been electrical in nature, and have generated electrical signals which are transmitted through one or more electrical transmission elements 26, in the form of wires or metallic strips, which extend through the umbilical cord 18 from the pressure sensor 16 to a termination point on an electric circuit board 28 located within the housing 20 of the pressure monitoring apparatus 14.
A pressure reference tube 30 also extends through the umbilical cord 18 from the pressure sensor 16 to an open-ended termination point located inside of the housing 20. The housing 20 is typically vented to the atmosphere surrounding the housing 20 through a vent 32, which may include internal structures to preclude the entry of fluid into the housing 20, or provide a flame arrester, where volatile fumes or liquids may be involved.
By virtue of this arrangement, ambient air pressure surrounding the housing 20 is communicated through the vent 32 and the open end of the pressure reference tube 30 to the differential pressure sensor 16 located below the liquid level 10 in the tank 12. By sensing changes in differential pressure, the pressure sensor 16 is able to provide an output to the internal circuit 28 indicative of the fluid level 10 in the tank 12. In some instances, the circuitry 28 has included provisions for also determining changes in the level of the liquid 10 in the tank 12 over time, as a function of inputs received from the differential pressure sensor 16.
While such an approach works well in many applications, under certain conditions, and with certain fluids such as propane, it is desirable to have a differential pressure sensor monitor a difference in pressure between an upper, gas-filled portion 34 of the tank 12 and a lower, liquid-filled portion 36 of the tank 12.